The invention relates to a process for the removal of ammonium carbamate from an aqueous urea solution containing ammonium carbamate such as the urea-synthesis solution resulting from the synthesis of urea from ammonia and carbon dioxide at elevated temperature and pressure. The aqueous urea solution is made to flow down a wall as thin film at a pressure of 120-180 atmospheres at a temperature of 170.degree.-200.degree. C. in counter-current relation to a gaseous stripping agent, while heat is applied, and the stripped aqueous urea solution and the resulting gaseous mixture formed in stripping are discharged by way of a liquid collecting chamber and a gas collecting chamber, respectively.
A stripping process of this type is described in U.S. Pat. No. 3,356,723 wherein a urea-synthesis solution is contacted with gaseous carbon dioxide in counter-current flow with heating, preferably at the urea-synthesis pressure. It is also stated that such a stripping process can be carried out by means of a gaseous ammonia stripping agent, although not as advantageously.
By adding gaseous carbon dioxide to the gas phase at equilibrium with the solution being stripped, the composition of the gas phase is changed so that it is no longer in equilibrium with the solution. As a result, part of the ammonium carbamate contained in the liquid phase decomposes and the ammonia and carbon dioxide thus formed pass into the gas phase in order to restore the heterogeneous equilibrium. This desorption results in the equilibrium between ammonium carbamate, urea and water in the liquid phase being disturbed, which in turn causes part of the urea that has already formed to hydrolyze back into the starting products ammonia and carbon dioxide.
The rate of such hydrolysis of urea increases with temperature. For this reason the temperature at which the stripping treatment is effected in the known process is kept comparatively low, and the retention time of the urea-synthesis solution in the stripping zone is kept relatively short by use of a wetted wall heat exchanger. Nevertheless, hydrolysis of urea does occur and this has an unfavorable influence on the conversion to urea, and consequently on the efficiency of the overall process for preparing urea in which the stripping treatment is used.
Another secondary reaction that takes place during and as a consequence of the stripping treatment is the formation of biuret, a compound that will cause damage to plants when contained in urea fertilizer in too high a concentration. For this reason, the biuret content of solid urea used for fertilizer purposes must not exceed 2.5% by weight, and should be no higher than 0.3% by weight in fertilizer solutions that are to be applied to the leaves. The biuret content of the final product is also a function of both time and temperature, increasing with increases in temperature and the time during which a urea-synthesis solution is treated to decompose unconverted ammonium carbamate and desorb the resulting ammonia and carbon dioxide.
Thus, in order to reduce both the hydrolysis of urea and the conversion of urea into biuret, it is desirable to minimize the time at which the urea-synthesis solution is exposed to high temperatures. On the other hand, it is necessary to maintain the urea-synthesis solution at an elevated temperature for some period of time in order to effectively decompose the ammonium carbamate and desorb the resulting ammonia and carbon dioxide.
It is therefore an object of the present invention to provide a process and apparatus whereby it is possible to minimize the undesirable hydrolysis of urea and formation of biuret while at the same time sufficiently decomposing the ammonium carbamate and removing it from the aqueous urea solution.